This invention relates to dielectric band elimination filters which use a dielectric resonator apparatus having series-connected resonant capacitors formed with a single dielectric block. This invention also relates to such filters, of which mutually adjacent pairs of resonators of the resonator apparatus do not couple at the trap frequencies.
Consider, for example, a dielectric band elimination filter which comprises two resonators and of which the equivalent circuit diagram is as shown in FIG. 13 wherein R.sub.1 and R.sub.2 are the two resonators, C.sub.e1 and C.sub.e2 are series-connected resonant capacitors of the resonators R.sub.1 and R.sub.2, and .phi. is a quarter-wavelength phase circuit. FIG. 14 is a similar equivalent circuit diagram of another dielectric band elimination filter which is different from the one shown by the diagram of FIG. 13 wherein the functions of the phase circuit are performed by an inductor L and stray capacitors C.sub.3 and C.sub.4. Components which are substantially identical to those shown in FIG. 13 are indicated in FIG. 14 by the same symbols.
A dielectric filter having such an equivalent circuit diagram has conventionally been produced, as shown in FIG. 15, by using two each of discrete resonators R.sub.1 and R.sub.2, series-connected capacitors C.sub.e1 and C.sub.e2 and connector terminals 82a and 82b, together with a substrate 80, a shield cover 81 and either a coil L.sub.1 or a printed inductor L.sub.2.
Prior art dielectric band elimination filters have many components to assemble for the production, requiring many production steps, and are difficult to make compact because the numbers of resonators, capacitors and connector terminals increase in proportion to the number of stages. It is therefore an object of this invention to provide a compact dielectric band elimination filter with a reduced number of component parts which can be assembled in a reduced number of production steps.
Consider, next, another band elimination filter, of which the equivalent circuit diagram is as shown in FIG. 16, including three resonators R.sub.1, R.sub.2, R.sub.3, three series-connected resonant capacitors C.sub.e, three stray capacitors C.sub.s and two connecting inductors L.sub.1 and L.sub.2. If such a filter is built by using three discrete resonators R.sub.1 -R.sub.3, as conventionally done, there is a one-to-one correspondence established between the individual resonant frequencies of these discrete resonators R.sub.1 -R.sub.3 and the trap frequencies after the band elimination filter has been constructed. In other words, adjacent resonators do not affect characteristics of each other.
If a filter, shown by the equivalent circuit diagram of FIG. 16, is built with three resonators unistructurally formed inside a single dielectric block, there may no longer be a one-to-one correspondence between the resonant frequencies of the individual resonators and the trap frequencies of the completed band elimination filter. This lack of correspondence is illustrated in FIG. 17 wherein the solid line indicates the original band elimination characteristic having three trap frequencies corresponding to the three resonators. If the length of one of the resonators is changed such that its trap (attenuation pole) frequency is changed from f.sub.1 on this solid line to f.sub.2, the trap frequencies of the other two resonators in the block are thereby affected and also undergo changes, and the trap characteristic may look as shown by the broken line in FIG. 17.
Accordingly, it has been difficult to individually adjust the characteristics of the resonators thus formed, and practically usable band-elimination filters of this type could not be produced. This was because both even and odd modes of oscillations are generated inside the dielectric block such that mutually adjacent pairs of the resonators are coupled to each other, affecting performance characteristics of each other.
It is therefore another object of this invention to provide a dielectric filter of the type having a plurality of resonators formed inside a single dielectric block, of which the trap frequencies can be adjusted independently, without affecting the characteristics of the adjacent resonators.